The present invention relates, in general, to electronics and, more particularly, to low dropout voltage regulators.
There are various known types of voltage regulators for power management systems, including both linear regulators and switch mode regulators. One particularly useful type of regulator is referred to as a low dropout (LDO) voltage regulator. LDO voltage regulators can operate correctly even when the input voltage is only about 0.5 volts higher than the regulated output voltage, and thus the LDO voltage regulators are particularly useful for high efficiency power management systems like battery operated devices. A typical LDO voltage regulator includes a voltage reference such as a bandgap voltage reference circuit, an error amplifier, and an output voltage divider. The error amplifier changes the output voltage to make the divided output voltage equal to the reference voltage, and typically includes a pass transistor between the input and output voltage terminals.
Because LDO voltage regulators are useful in such a large number of portable applications, semiconductor manufacturers have sought ways to reduce their sizes while maintaining their ability to control large output circuit elements such as a pass transistor. Techniques for reducing the sizes of LDO voltage regulators have resulted in a large increase in their quiescent currents, which reduces their suitability for portable applications because of an increased power drain.
Accordingly, it would be advantageous to have an LDO voltage regulator and method for regulating an output voltage in which the LDO voltage regulator is configured to have a small form factor with a reduced quiescent current. It would be of further advantage for the LDO voltage regulator and method to be cost efficient to implement.
For simplicity and clarity of illustration, elements in the figures are not necessarily to scale, and the same reference characters in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or anode of a diode, and a control electrode means an element of the device that controls current flow through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain N-channel or P-channel devices, or certain N-type or P-type doped regions, a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with embodiments of the present invention. It will be appreciated by those skilled in the art that the words during, while, and when as used herein are not exact terms that mean an action takes place instantly upon an initiating action but that there may be some small but reasonable delay, such as a propagation delay, between the reaction that is initiated by the initial action. The use of the word approximately, about, or substantially means that a value of an element has a parameter that is expected to be very close to a stated value or position. However, as is well known in the art there are always minor variances that prevent the values or positions from being exactly as stated. It is well established in the art that variances of up to about ten percent (10%) (and up to twenty percent (20%) for semiconductor doping concentrations) are regarded as reasonable variances from the ideal goal of exactly as described.
It should be noted that a logic zero voltage level (VL) is also referred to as a logic low voltage and that the voltage level of a logic zero voltage is a function of the power supply voltage and the type of logic family. For example, in a Complementary Metal Oxide Semiconductor (CMOS) logic family a logic zero voltage may be thirty percent of the power supply voltage level. In a five volt Translator-Translator Logic (TTL) system a logic low voltage level may be about 0.8 volts, whereas for a five volt CMOS system, the logic zero voltage level may be about 1.5 volts. A logic one voltage level (VH) is also referred to as a logic high voltage level and, like the logic zero voltage level, the logic high voltage level also may be a function of the power supply and the type of logic family. For example, in a CMOS system a logic one voltage may be about seventy percent of the power supply voltage level. In a five volt TTL system a logic one voltage may be about 2.4 volts, whereas for a five volt CMOS system, the logic one voltage may be about 3.5 volts.